Characteristic of In Situ Stress and Its Control on the Coalbed Methane Reservoir Permeability in the Eastern Margin of the Ordos Basin, China

Coalbed methane (CBM) development faces many challenges, among which in situ stress and permeability are two of the most important and fundamental factors. Knowledge of the characteristics of these factors is crucial to CBM exploration and development. Based on measured injection/falloff and in situ stress well test data of 55 CBM wells in the eastern margin of the Ordos Basin, correlations between parameters including initial reservoir pressure, in situ stress, lateral stress coefficient, well test permeability, and burial depth were determined. The distribution of in situ stress was analyzed systematically and its influence on permeability was also addressed. The results indicate that the maximum horizontal principal stress (sigma (H) 10.13-37.84 MPa, average 22.50 MPa), minimum horizontal principal stress (sigma (h) 6.98-26.88 MPa, average 15.04 MPa) and vertical stress (sigma (v) 12.30-35.72 MPa, average 22.48 MPa) all have positive correlations with coal burial depth. Stress ratios (sigma (H)/sigma (h), sigma (H)/sigma (v), and sigma (h)/sigma (v)) and lateral stress coefficient slowly attenuated with depth. With increase of horizontal principal stresses, coal reservoir permeability (0.01-3.33 mD, average 0.65 mD) decreases. The permeability variation is basically consistent with change of stress state at a certain burial depth, the essence of which is the deformation and destruction of coal pore structures under the action of stresses. Three types of stress fields exist in the area: in the shallow coal seam at burial depths < 700 m, the horizontal principal stress is dominant, revealing a strike slip regime (sigma (H) > sigma (v) > sigma (h)), with average permeability 0.89 mD; from 700 to 1000 m depths, there is a stress transition zone (sigma (H) ae sigma (v) > sigma (h)) with average permeability 0.73 mD; in the deep coal seam with burial depths > 1000 m, the vertical principal stress is dominant, demonstrating a normal stress regime (sigma (v) > sigma (H) > sigma (h)) with average permeability 0.11 mD.